The invention relates to a method of generating a pulse code modulation (PCM) code set for encoding data by a PCM encoder in a data transmission system. The data transmission system contains a digital modem having the PCM encoder, a digital signal transmission path, an analogue signal transmission path having a central office line interface portion, and an analogue modem for generating the PCM code set for encoding data by the PCM encoder. The invention further relates to an apparatus for carrying out such a method.
Recently, substantial progress has been made in increasing the data transmission rates when transmitting data over. conventional analogue telephone lines. The International Telecommunications Union (ITU) has promulgated and published various recommendations, such as V.32, V.32bis, or V.34, that are concerned with data transmission over telephone lines. These recommendations are all based on a transmission technique called quadrature amplitude modulation (QAM). QAM has proven advantageous for the plain old telephone system (POTS) environment.
Nevertheless, the network of telephone systems has undergone massive changes in that the network is nowadays almost entirely digital. The analogue signals originating from a first subscriber modem are converted at the central office of the subscriber to digital representations that are carried through the digital telephone network. At the central office of a second subscriber, the digital signals are converted back into analogue signals to be driven into a subscriber line of the second subscriber. The modem of the second subscriber interprets the analogue signals on the analogue subscriber line by demodulating the QAM signals produced by the modem of the first subscriber. The same way of data communication is carried out in the reverse direction.
Increasingly more subscribers are connected to the telephone network through a digital subscriber interface, such as ISDN. Thus, many data connections are established between a first subscriber having an analogue network interface and a second subscriber having a digital network interface. In many cases, the second subscriber will be an internet service provider. In order to optimize data transmission over such heterogeneous communication channels, various proposals have been made in the recent past. One such proposal is known from International Patent Application WO 96/18261.
The proposal is based on the idea that the transmission rate in a heterogeneous communication channel from the digital subscriber to the analogue subscriber, maybe raised by using the PCM coding technique instead of the former QAM modulation techniques. The PCM coding technique uses a plurality of signal levels for encoding data symbols (each data symbol containing multiple bits). These signal levels are again recognized by the receiving modem that is then able to decode the data symbol encoded into the signal levels.
Further, the ITU has published a draft for a new recommendation V.90 on May 6, 1998. The new recommendation also relies on the PCM coding technique for the transmission of data from the digital subscriber to the analogue subscriber. Draft recommendation V.90 in terms of its PCM coding scheme depends on ITU-T recommendation G.711 describing Pulse Code Modulation (PCM) of Voice Frequencies which is generally applied in telephone networks throughout the world when converting analogue signal amplitude values into numeric representations thereof, and vice versa. G.711 recommends two PCM coding schemes generally known as xcexc-law, which is applied in North American telephone networks, and A-law, which is applied in most other telephone networks. Both coding schemes have in common that they have a logarithmic coding characteristic, i.e. the lower the signal amplitude value to be encoded, the more fine-grain the available PCM codes. Such logarithmic coding characteristic has been found to be particularly advantageous for encoding analogue voice signals at minimum distortion.
Recommendation G.711 makes available 256 PCM codes (or U-codes as they are called in the draft of V.90) which are grouped into eight positive and eight negative segments (or U-chords as they are called in the draft of V.90). Each PCM code is encoded using eight bits. Due to power restrictions on the analogue telephone line and due to line impairments, the analogue modem (according to the terminology used in the draft to V.90) receiving analogue amplitude values is unable to discriminate between all 256 available PCM codes. Therefore, a reduced set of PCM codes is determined for encoding data symbols during the set-up of a data communication channel under real world conditions. This accordingly lowers the data transmission rate down from the maximum theoretical possible value of 64 kbit/s such that it is not above 56 kbit/s.
Measurements on existing V.90 modems show that the actual data rate for the PCM transmission path from the digital modem to the analogue modem is even considerably below the expected 56 kbit/s. Published, European Patent Application EP 0 833 481 A1 suggests that an attenuation in any analogue transmission path has to be compensated for. However, the European Patent Application EP 0 833 481 fails to set out in detail how such compensation is supposed to be accomplished.
It is accordingly an object of the invention to provide a method of and an apparatus for generating PCM code sets that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, in which the data transmission rate in a communications system using a PCM data encoding technique is enhanced.
With the foregoing and other objects in view there is provided, in accordance with the invention, an improved method of generating a pulse code modulation code set for encoding data with a PCM encoder in a data transmission system having a digital modem with the PCM encoder, a digital signal transmission path, an analogue signal transmission path with a central office line interface portion, and an analogue modem, the improvement which includes:
determining a minimum signal level distance between adjacent PCM codes of the PCM code set, the minimum signal level distance enabling discrimination by the analogue modem for determining a line interface attenuation of the central office line interface portion; and
generating the PCM code set on a basis of a power limit computed by adding the line interface attenuation to a central office line interface power limit.
In the invention, the minimum signal level distance between adjacent PCM codes of the PCM code set is determined. The minimum signal level distance is the level distance between successive symbols that can be safely discriminated by the analogue modem of the communications system. Then, the line interface attenuation of the central office line interface portion of the communications system is determined. Eventually, a PCM code set is generated on the basis of a power limit computed by adding line interface attenuation to the central office line interface power limit.
Output power limits are generally to be observed at the line interface terminal output to the subscriber telephone line. Telephone operators set their own power limits depending on various properties of the network. For this reason, the central office line interface for each subscriber line includes attenuation capabilities to make sure that power limits are within specified limits. The invention compensates for the attenuation introduced by the attenuators at the central office line interface. The digital modem of the communications system may then generate a PCM code set that is close to the power limit measured at the output of the central office line interface to the subscriber line. This allows for more PCM codes to be included in a particular PCM code set and thus results in higher data transmission rates from the digital modem to the analogue modem.
In a preferred embodiment of the invention, the line interface attenuation is computed by measuring the total attenuation of the signal path from the digital modem to the analogue modem and by subtracting therefrom an attenuation value caused by the line interface of the analogue modem and an estimate of an attenuation value caused by a line portion of the analogue signal transmission path. The result is compared with values contained in a look-up table. The look-up table includes values of expected attenuation values for the line interface attenuation prescribed by various network operators. The nearest table value is taken as a basis for computing an adapted PCM code set according to the invention. By a measurement of attenuation values and a comparison, it is possible to substantially improve the PCM coding efficiency and thus the obtainable data transmission rate.
In a further preferred embodiment, output power limits of the central office line interface are stored in a look-up table. In yet another preferred embodiment, the method is carried out by the analogue modem. This is particularly advantageous in a data transmission system according to the would-be recommendation V.90. Signals transferred in phase two of communication start-up signals and sequences can be utilized to measure the total attenuation to which a signal from the digital modem to the analogue modem is subjected.
An embodiment of the analogue modem according to the invention includes means for determining a minimum signal level distance between adjacent PCM codes of the PCM code set. The signal minimum signal level distance enables discrimination by the analogue modem. The analogue modem further includes means for determining a line interface attenuation of the central office line interface portion and a PCM code set generator for generating the PCM code set on the basis of the power limit computed by adding the line interface attenuation to a central office line interface power limit.
A preferred embodiment of the modem further includes a table including line interface attenuation values respectively prescribed by a plurality of central office operators. A still further preferred embodiment of the modem includes a table including power limit values respectively prescribed by a plurality of central office operators.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of and an apparatus for generating PCM code sets, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.